This invention relates to inkjet printers and, more particularly, to a monolithic printhead for an inkjet printer.
Inkjet printers typically have a printhead mounted on a carriage that scans back and forth across the width of a sheet of paper feeding through the printer. Ink from an ink reservoir, either on-board the carriage or external to the carriage, is fed to ink ejection chambers on the printhead. Each ink ejection chamber contains an ink ejection element, such as a heater resistor or a piezoelectric element, which is independently addressable. Energizing an ink ejection element causes a droplet of ink to be ejected through a nozzle for creating a small dot on the medium. The pattern of dots created forms an image or text.
Additional information regarding one particular type of printhead and inkjet printer is found in U.S. Pat. No. 5,648,806, entitled, xe2x80x9cStable Substrate Structure For A Wide Swath Nozzle Array In A High Resolution Inkjet Printer,xe2x80x9d by Steven Steinfield et al., assigned to the present assignee and incorporated herein by reference.
As the resolutions and printing speeds of printheads increase to meet the demanding needs of the consumer market, new printhead manufacturing techniques and structures are required.
Described herein is a monolithic printhead formed using integrated circuit techniques.
A silicon substrate has formed on its top surface a thin polysilicon layer in the area in which a trench is to be later formed in the substrate. The edges of the polysilicon layer align with the intended placement of ink feed holes leading into ink ejection chambers. Thin film layers, including a resistive layer, are then formed on the top surface of the silicon substrate. The thin film layers include oxide layers formed over the polysilicon layer. The various layers are etched to provide conductive leads to the heater resistor elements. Piezoelectric elements may be used instead of the resistive elements.
At least one ink feed hole is partially formed through the thin film layers for each ink ejection chamber, leaving the oxide layers over the polysilicon layer in the ink feed hole areas.
An orifice layer is formed on the top surface of the thin film layers to define the nozzles and ink ejection chambers. In one embodiment, a photo-definable material is used to form the orifice layer.
A trench mask is formed on the bottom surface of the substrate. A trench is etched (using, for example, TMAH) through the exposed bottom surface of the substrate. When the substrate is etched to the polysilicon layer, the TMAH rapidly etches away the polysilicon sandwiched between the silicon substrate and the oxide layers, creating a gap between the silicon substrate and the oxide layers. This gap exposes fast etch planes of the silicon. Such fast etch planes may be, for example, (110) and others. The TMAH then rapidly etches the silicon substrate along the etch planes, thus aligning the edges of the trench with the polysilicon edges. The lateral (in the plane of the wafer) trench etch rate during this rapid etch has been shown in simulations to be 100 microns or more per hour as compared with the lateral component of purely (111) plane etching, which is usually 2-6 microns per hour. The rapid lateral etch rate is almost twice as fast as the vertical etch rate along the  less than 100 greater than  direction.
A wet etch is then performed using a buffered oxide etch (BOE) solution. The etchant enters the ink chambers through the nozzles and etches the exposed oxide layers in the ink feed hole areas from the topside. The oxide layers exposed by the trench are also etched from the underside during the same wet etching step. Thus, the wet etching, without the use of any masks, quickly etches the exposed oxide layers from the topside and underside. The BOE will completely etch through the exposed oxide layers forming ink feed holes through the thin film layers. The trench is aligned with the ink feed holes due to the polysilicon layer.
This process allows some misalignment of the trench mask without affecting the final trench dimensions.
The resulting fully integrated thermal inkjet printhead can be manufactured to a very precise tolerance since the entire structure is monolithic, meeting the needs for the next generation of printheads.
The process may be used to form openings in devices other than printheads.